1. Field of the Invention
This invention relates to methods for fabricating solid state emitters and in particular methods for tuning the emission characteristics of light emitting diodes coated by a conversion material.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; Cree® Inc. EZBright™ LEDs, XThin™ LEDs, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the wavelength of some of the LED's blue light, changing its color to yellow. In other devices the light can be converted to green, yellow, orange or red or a combination thereof. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted. The LED emits both blue and yellow and/or red light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
One conventional method for coating an LED with a phosphor layer utilizes a syringe or nozzle for injecting a phosphor mixed with epoxy resin or silicone polymers over the LED. Using this method, however, it can be difficult to control the phosphor layer's geometry and thickness. As a result, light emitting from the LED at different angles can pass through different amounts of conversion material, which can result in an LED with non-uniform color temperature as a function of viewing angle.
Another conventional method for coating an LED is by stencil printing, which is described in European Patent Application EP 1198016 A2 to Lowery. Multiple light emitting semiconductor devices are arranged on a substrate with a desired distance between adjacent LEDs. The stencil is provided having openings that align with the LEDs, with the holes being slightly larger than the LEDs and the stencil being thicker than the LEDs. A stencil is positioned on the substrate with each of the LEDs located within a respective opening in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
Like the syringe method above, using the stencil method can be difficult to control the geometry and layer thickness of the phosphor containing polymer. The stenciling composition may not fully fill the stencil opening such that the resulting layer is not uniform. The phosphor containing composition can also stick to the stencil opening which reduces the amount of composition remaining on the LED. The stencil openings may also be misaligned to the LED. These problems can result in LEDs having non-uniform color temperature and LEDs that are difficult to consistently reproduce with the same or similar emission characteristics.
Many of the above coating processes focus on coating at the LED chip level or at the package level. There has been recent interest in coating LEDs with a phosphor at the wafer level instead of the chip level to reduce the cost and complexity of fabrication. With these approaches one challenge is accessing the wire bond pad on the device after the coating process. Accessing the wire bond by standard wafer fabrication techniques is difficult with typical silicone binding material, as well as other binder materials such as epoxies or glass. Silicones are not compatible with commonly used wafer fabrication materials such as acetone, as well as some developers and resist strippers. This can limit the options and choices of process steps for the particular silicones. Silicones are also cured at high temperature (greater than 150° C.), which is beyond the glass transition temperature of commonly used photoresists. Cured silicone films with phosphor are also difficult to etch and have a very slow etch rate in chlorine and CF4 plasma, and wet etching of cured silicones is typically inefficient.
LEDs across a wafer can also have different emission characteristics or color spread. FIG. 1 shows one example of a wavelength emission map 10 for a wafer of blue emitting LEDs showing wavelength variations across the wafer, and each wafer can have its own unique emission map. In the map shown, the wavelength distribution is approximately 450 to 460 nm, although other wafers can experience different distributions in different wafer areas. This distribution can result from different factors such as variations in the epitaxial material during growth of the LEDs, or from variations in the flatness (i.e. bow) of the growth substrate.
The wafer can be coated with a conversion material (i.e. phosphor) using one of the methods described above, and FIG. 2 shows a conversion material thickness map 20 following coating. In some fabrication processes the coating can be planarized using known methods. The thickness of the coating can vary across the wafer due to different factors such as variations in the thickness of the underlying wafer and in planarizing variations. In the embodiment shown the wafer experiences a total thickness variation of approximately 3 μm. The wavelength emission variations of the LEDs and thickness variations of the conversion material across the wafer can result in a spread of emission wavelengths or color points of the LED chips singulated from the wafer. This spread can exacerbate by phosphor loading variations or concentrations across the wafer.
The human eye is relatively sensitive to variations in emission wavelengths and can detect relatively small differences in emission wavelengths or color. Perceptible variations in color emitted by packages designed to emit a single color of light can reduce customer satisfaction and reduce overall acceptance of LED packages for commercial uses. In an effort to provide LEDs that emit light of the same or similar wavelength, the LEDs can be tested and sorted by color or brightness. This process is generally known in the art as binning. Each bin typically contains LEDs from one color and brightness group and is typically identified by a bin code. White emitting LEDs can be sorted by chromaticity (color) and luminous flux (brightness). Color LEDs can be sorted by dominant wavelength, and luminous flux, or in the case of certain colors such as royal blue, by radiant flux. LEDs can be shipped, such as on reels, containing LEDs from one bin and are labeled with the appropriate bin code.
There is also interest in optimizing the emission efficiency of LEDs that are coated at the wafer level. For certain LEDs it is common to provide more than one phosphor mixed in the phosphor coating. For warm white emitting LEDs a blue emitting LED can be coated with a yellow and red phosphor mixture, such that the end LED package emits a warm white light combination of blue, yellow and red. These different phosphors can have different excitation and re-emission spectrums, and FIG. 3 is a graph 30 showing the excitation spectrum 32 of a red phosphor and the emission spectrum 34 of a yellow phosphor. The lower portion of red phosphor excitation spectrum 32 overlaps with upper portion of the yellow phosphor emission spectrum 34. That is, the upper wavelengths for emission from the yellow phosphor can be the same as the lower wavelengths that are absorbed by the red phosphor. This can result in a certain percentage of light that is absorbed by the yellow phosphor and re-emitted as yellow light being absorbed again by the red phosphor. Each phosphor absorption and re-emission results in Stokes losses and the overlap of the spectrum as shown in FIG. 3 can reduce the overall emission efficiency by greater than 30%.
When the phosphor mixture is applied to the LED it can be in direct contact with or close to the LED's semiconductor layers such that heat from the LED spreads to the phosphor. The different phosphors can react differently to the LED heat and some can degrade from the heat. As one example, certain yellow phosphors such as YAG can be more sensitive to heat compared to red phosphors and the efficacy of these YAG phosphors can degrade over time.